Gasoline prices nationwide continued their downward spiral, dropping an average of more than half a cent during the past two weeks, an oil industry analyst said Sunday.

Traditionally low seasonal demand by motorists and a weak crude oil market combined to lower the average pump price of a gallon of gasoline by .58 cents to 99.03 cents during the period from Dec. 2 to Dec. 16, said Trilby Lundberg, publisher of the Lundberg Letter.Lundberg's biweekly survey of 12,000 gasoline stations found that the average price of all grades of gas at both self-serve and full-serve pumps was nearly a cent lower than the price of 99.87 cents a gallon in December 1987.

"It's possible that prices could decline further in the early weeks of the new year because wholesale prices to retailers dropped more than pump prices did."